1. Field of the Invention
The present invention relates to a valve lifter for a three-dimensional cam used in an internal combustion engine and a variable valve operating apparatus using the valve lifter.
2. Description of the Related Art
There is a known variable valve operating apparatus capable of varying the timing for opening and closing an intake valve or an exhaust valve of an internal combustion engine in accordance with a driving state of the internal combustion engine. As one example of the variable valve operating apparatus, there is a known apparatus for adjusting the opening and closing timing of a valve by varying the lift amount of a valve 1003 using a three-dimensional cam 1002 capable of moving in a direction of a rotation axis as shown in FIG. 38 (Japanese Patent Application Laid-Open No. HEI 10-121926).
In the variable valve operating apparatus using such a three-dimensional cam, the incline angle of a cam surface 1002a is varied as the three-dimensional cam rotates. Therefore, a guide groove 1005 extending in parallel with the rotation direction (direction of the arrow S in the drawing) of the three-dimensional cam is formed on a top surface 1004a of a valve lifter 1004. A semicolumnar rocking follower 1006 capable of rocking in accordance with variation of the incline angle of the cam surface 1002a is fitted in the guide groove 1005 for enhancing the durability by maintaining sufficient contact between the three-dimensional cam 1002 and the valve lifter 1004.
In the aforementioned structure, the cam surface 1002a of the three-dimensional cam 1002 applies pressure to the valve lifter 1004 obliquely through the rocking follower 1006. Therefore, a strong moment acts on the valve lifter 1004 about its axis which is caused to rotate in a lifter bore 1009a provided in a cylinder head 1009 of the internal combustion engine. The aforementioned rotation of the valve lifter 1004 may change the direction of the rocking follower 1006 undesirably. Therefore, the valve lifter 1004 is provided with a projection 1004c at its outer peripheral surface as a detent mechanism for the valve lifter 1004 such that the projection 1004c is brought into engagement with a groove formed in an inner peripheral surface of the lifter bore 1009a in its axial direction. This mechanism allows the valve lifter 1004 to slide within the lifter bore 1009a in its axial direction but not to rotate, thus maintaining the direction of the rocking follower 1006.
However, there has been a recent trend to decrease the thickness of the valve lifter 1004 in order to reduce the weight of the internal combustion engine. This may restrict a position for mounting the projection 1004c or its shape as well.
For example, as the detent projection cannot be force fitted to the outer peripheral surface 1004b as a thin portion, it is necessary to mount the projection by welding that requires more time than the force fitting. When the projection is mounted on the thin portion by welding, the inside thereof may be deformed to give an adverse effect on roundness of the valve lifter 1004. Further, as a largesized projection cannot be mounted, the surface pressure applied to the groove to be engaged with the projection is increased. It is, thus, probable to threaten deteriorated durability.
A similar problem may be raised when forming a detent groove on the valve lifter 1004. Since a groove with sufficient depth cannot be formed in the thin portion, the area contacting the projection inserted from the cylinder head is reduced to increase the surface pressure. As A result, a detent mechanism having sufficient durability cannot be obtained.
In order to form the detent projection having sufficient contacting area, there has been no alternative but to mount a small-sized projection on a relatively thick portion near the top surface 1004a of the valve lifter. The degree of freedom in the detent mechanism design, thus, is extremely lowered.